比较运算符中使用NULL
mysql> select 1>NULL; +--------+ | 1>NULL | +--------+ | NULL | +--------+ 1 row in set (0.00 sec) mysql> select 1<NULL; +--------+ | 1<NULL | +--------+ | NULL | +--------+ 1 row in set (0.00 sec) mysql> select 1<>NULL; +---------+ | 1<>NULL | +---------+ | NULL | +---------+ 1 row in set (0.00 sec) mysql> select 1>NULL; +--------+ | 1>NULL | +--------+ | NULL | +--------+ 1 row in set (0.00 sec) mysql> select 1<NULL; +--------+ | 1<NULL | +--------+ | NULL | +--------+ 1 row in set (0.00 sec) mysql> select 1>=NULL; +---------+ | 1>=NULL | +---------+ | NULL | +---------+ 1 row in set (0.00 sec) mysql> select 1<=NULL; +---------+ | 1<=NULL | +---------+ | NULL | +---------+ 1 row in set (0.00 sec) mysql> select 1!=NULL; +---------+ | 1!=NULL | +---------+ | NULL | +---------+ 1 row in set (0.00 sec) mysql> select 1<>NULL; +---------+ | 1<>NULL | +---------+ | NULL | +---------+ 1 row in set (0.00 sec) mysql> select NULL=NULL,NULL!=NULL; +-----------+------------+ | NULL=NULL | NULL!=NULL | +-----------+------------+ | NULL | NULL | +-----------+------------+ 1 row in set (0.00 sec) mysql> select 1 in (null),1 not in (null),null in (null),null not in (null); +-------------+-----------------+----------------+--------------------+ | 1 in (null) | 1 not in (null) | null in (null) | null not in (null) | +-------------+-----------------+----------------+--------------------+ | NULL | NULL | NULL | NULL | +-------------+-----------------+----------------+--------------------+ 1 row in set (0.00 sec) mysql> select 1=any(select null),null=any(select null); +--------------------+-----------------------+ | 1=any(select null) | null=any(select null) | +--------------------+-----------------------+ | NULL | NULL | +--------------------+-----------------------+ 1 row in set (0.00 sec) mysql> select 1=all(select null),null=all(select null); +--------------------+-----------------------+ | 1=all(select null) | null=all(select null) | +--------------------+-----------------------+ | NULL | NULL | +--------------------+-----------------------+ 1 row in set (0.00 sec)
结论:任何值和NULL使用运算符(>、<、>=、<=、!=、<>)或者(in、not in、any/some、all)比较时,返回值都为NULL,NULL作为布尔值的时候,不为1也不为0。
准备数据
mysql> create table test1(a int,b int); Query OK, 0 rows affected (0.01 sec) mysql> insert into test1 values (1,1),(1,null),(null,null); Query OK, 3 rows affected (0.00 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> select * from test1; +------+------+ | a | b | +------+------+ | 1 | 1 | | 1 | NULL | | NULL | NULL | +------+------+ 3 rows in set (0.00 sec)
上面3条数据,认真看一下,特别是注意上面NULL的记录。
IN、NOT IN和NULL比较
IN和NULL比较
mysql> select * from test1; +------+------+ | a | b | +------+------+ | 1 | 1 | | 1 | NULL | | NULL | NULL | +------+------+ 3 rows in set (0.00 sec) mysql> select * from test1 where a in (null); Empty set (0.00 sec) mysql> select * from test1 where a in (null,1); +------+------+ | a | b | +------+------+ | 1 | 1 | | 1 | NULL | +------+------+ 2 rows in set (0.00 sec)
结论:当IN和NULL比较时,无法查询出为NULL的记录。
NOT IN 和NULL比较
mysql> select * from test1 where a not in (1); Empty set (0.00 sec) mysql> select * from test1 where a not in (null); Empty set (0.00 sec) mysql> select * from test1 where a not in (null,2); Empty set (0.00 sec) mysql> select * from test1 where a not in (2); +------+------+ | a | b | +------+------+ | 1 | 1 | | 1 | NULL | +------+------+ 2 rows in set (0.00 sec)
结论:当NOT IN 后面有NULL值时,不论什么情况下,整个sql的查询结果都为空。
EXISTS、NOT EXISTS和NULL比较
mysql> select * from test2; +------+------+ | a | b | +------+------+ | 1 | 1 | | 1 | NULL | | NULL | NULL | +------+------+ 3 rows in set (0.00 sec) mysql> select * from test1 t1 where exists (select * from test2 t2 where t1.a = t2.a); +------+------+ | a | b | +------+------+ | 1 | 1 | | 1 | NULL | +------+------+ 2 rows in set (0.00 sec) mysql> select * from test1 t1 where not exists (select * from test2 t2 where t1.a = t2.a); +------+------+ | a | b | +------+------+ | NULL | NULL | +------+------+ 1 row in set (0.00 sec)
上面我们复制了表test1创建了表test2。
查询语句中使用exists、not exists对比test1.a=test2.a,因为=不能比较NULL,结果和预期一致。
判断NULL只能用IS NULL、IS NOT NULL
mysql> select 1 is not null; +---------------+ | 1 is not null | +---------------+ | 1 | +---------------+ 1 row in set (0.00 sec) mysql> select 1 is null; +-----------+ | 1 is null | +-----------+ | 0 | +-----------+ 1 row in set (0.00 sec) mysql> select null is null; +--------------+ | null is null | +--------------+ | 1 | +--------------+ 1 row in set (0.00 sec) mysql> select null is not null; +------------------+ | null is not null | +------------------+ | 0 | +------------------+ 1 row in set (0.00 sec)
看上面的效果,返回的结果为1或者0。
结论:判断是否为空只能用IS NULL、IS NOT NULL。
聚合函数中NULL的坑
示例
mysql> select count(a),count(b),count(*) from test1; +----------+----------+----------+ | count(a) | count(b) | count(*) | +----------+----------+----------+ | 2 | 1 | 3 | +----------+----------+----------+ 1 row in set (0.00 sec)
- count(a)返回了2行记录,a字段为NULL的没有统计出来。
- count(b)返回了1行记录,为NULL的2行记录没有统计出来。
- count(*)可以统计所有数据,不论字段的数据是否为NULL。
再继续看
mysql> select * from test1 where a is null; +------+------+ | a | b | +------+------+ | NULL | NULL | +------+------+ 1 row in set (0.00 sec) mysql> select count(a) from test1 where a is null; +----------+ | count(a) | +----------+ | 0 | +----------+ 1 row in set (0.00 sec)
上面第1个sql使用is null查询出了结果,第2个sql中count(a)返回的是0行。
结论:count(字段)无法统计字段为NULL的值,count(*)可以统计值为null的行。
NULL不能作为主键的值
mysql> create table test3(a int primary key,b int); Query OK, 0 rows affected (0.01 sec) mysql> insert into test3 values (null,1); ERROR 1048 (23000): Column 'a' cannot be null
上面我们创建了一个表test3,字段a未指定不能为空,插入了一条NULL的数据,报错原因:a 字段的值不能为NULL,我们看一下表的创建语句:
mysql> show create table test3; +-------+------------+ | Table | Create Table | +-------+------------+ | test3 | CREATE TABLE `test3` ( `a` int(11) NOT NULL, `b` int(11) DEFAULT NULL, PRIMARY KEY (`a`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 +-------+------------+ 1 row in set (0.00 sec)
从上面的脚本可以看出,当字段为主键的时候,字段会自动设置为not null。
结论:当字段为主键的时候,字段会自动设置为not null。
看了上面这些还是比较晕,NULL的情况确实比较难以处理,容易出错,最有效的方法就是避免使用NULL。所以,强烈建议创建字段的时候字段不允许为NULL,设置一个默认值。
总结
- NULL作为布尔值的时候,不为1也不为0
- 任何值和NULL使用运算符(>、<、>=、<=、!=、<>)或者(in、not in、any/some、all),返回值都为NULL
- 当IN和NULL比较时,无法查询出为NULL的记录
- 当NOT IN 后面有NULL值时,不论什么情况下,整个sql的查询结果都为空
- 判断是否为空只能用IS NULL、IS NOT NULL
- count(字段)无法统计字段为NULL的值,count(*)可以统计值为null的行
- 当字段为主键的时候,字段会自动设置为not null
- NULL导致的坑让人防不胜防,强烈建议创建字段的时候字段不允许为NULL,给个默认值
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